1. Field of the Invention
The invention is related to the field of seismic energy mapping of earth formations. More specifically, the invention is related to methods of determining geologic interconnection of an earth formation reservoir between wellbores by measuring seismic energy transmission properties of the earth formations between wellbores.
2. Description of the Related Art
Subterranean earth formations sometimes contain zones permeable to fluid flow called reservoirs. Wellbores are drilled into reservoirs for extracting the fluids, which can include commercially useful materials such as petroleum. Wellbore operators are particularly concerned about the geologic structure of reservoirs, as this structure affects the placement of the wellbores used to extract the fluids.
Various methods are known in the art for inferring the geologic structure of the reservoir from measurements made at the earth's surface, such as reflection seismic surveying. Other methods include correlation between wellbores of measurements made from within the wellbores, these measurements being made from various types of well logging instruments known in the art.
The methods of well logging and seismic surveying generally are not able to determine whether a reservoir is geologically continuous between two particular wellbores. Knowledge of the existence of geologic continuity may be important to the wellbore operator to properly place any additional wellbores. A method of mapping geologic continuity is described in U.S. Pat. No. 5,144,590 issued to Chon. The method disclosed in the Chon '590 patent includes inserting a seismic energy source into one wellbore which penetrates the reservoir, and measuring the seismic energy which travels to receivers placed in another wellbore which also penetrates the reservoir. The spectral characteristics of the energy which reaches the receivers as it travels from the source can be analyzed to determine if the reservoir is geologically continuous between the wellbores.
A limitation to the method described in the Chon '590 patent is that it uses all the energy from the source to determine connectivity between wellbores. In certain cases the reservoir may be geologically connected between the wellbores but the fluid which is present in the reservoir between the wellbores may not be continuous. A connectivity map based on all the energy transmitted between wellbore might indicate geologic discontinuity in such a case even though the reservoir is hydraulically continuous between the wellbores.
Another limitation of the method in the Chon '590 patent can be illustrated by a case in which the mineral content of the formation from which reservoir is composed changes between the wellbores. In this case the method in the Chon patent may also indicate geologic discontinuity between the wellbores while again, in fact, the reservoir is hydraulically continuous between the wellbores.
What is needed is an extension of the method described in the Chon '590 patent which enables the user to determine geologic continuity even when there is a fluid content of mineralogical change in the reservoir between wellbores.